1. Technical Field
This invention relates to a zirconia sol, a method for the production thereof, a porous ceramic-producing slurry, and a porous ceramic product obtained by the use of the slurry. More particularly, it relates to a transparent zirconia sol useful as a coating agent for imparting various properties such as resistance to heat and resistance to abrasion to a substrate, as an inorganic binder capable of manifesting high adhesive strength, or as a material for the manufacture of a zirconia type fine composite powder and zirconia type fibers and a method for the production of the zirconia sol. It further relates to a porous ceramic-producing slurry containing the zirconia sol as an essential component thereof and a porous ceramic product obtained by the use of the slurry and, more specifically, a mold of a porous ceramic material useful for precision casting of such highly active metals as titanium alloys.
2. Background Art
In the methods heretofore used for the production of a zirconia sol from an aqueous solution of a zirconium salt, a method which comprises causing the aqueous solution of a zirconium salt to react with a basic substance thereby inducing formation of a precipitate and deflocculating the precipitate with an acid and a method which comprises pouring into the aqueous solution of a zirconium salt a basic substance of a pH value such as to avoid the occurrence of a precipitate thereby giving rise to a sol have been renowned. Besides, a method which produces a zirconia colloidal sol by heating the aqueous solution of a zirconium salt under normal pressure or under increased pressure thereby causing hydrolysis of the aqueous solution has been known to the art. The sol which is obtained by any of these methods is subsequently purged of ions.
The zirconia sol obtained as described above is used as a coating agent which is applied in the form of a coating to the surface of a substrate of a metallic substance, glass, or plastic substance for the purpose of imparting to the substrate such properties as resistance to heat, resistance to abrasion, resistance to chemicals, insulation, reflection proof, absorption of ultraviolet light, and rigidity at raised levels. It is used, in the field of inorganic binders, as a molding binder serving the purpose of impregnating porous refractory materials and as a casting binder for metallic titanium type cast products. It is further used as a raw material for piezoelectric materials, electroconductive paste, electronic parts such as a humidity sensor and a temperature sensor, catalysts, and other composite materials.
The zirconia sols which are produced by the conventional methods described above, however, have unduly low concentrations and suffer from inferior transparency due to excessively large diameters of sol particles. When they are used as a coating agent or an inorganic binder, therefore, the properties they manifest to a certain extent at all are not fully satisfactory in the field requiring a coating agent to form a film of large thickness and a binder to exhibit high adhesive strength.
Where a zirconia sol is used as a coating agent or a binder particularly at a high temperature, it is preferable to incorporate therein such an element as calcium, magnesium, or yttrium to serve as a stabilizer for preventing variation of the volume thereof due to transformation of the phase of zirconia. The zirconia sol which incorporates therein such an element as mentioned above, however, has the disadvantage that it is deficient in stability to withstand the influence of aging as evinced by the susceptibility to gelling and flocculation. For the prevention of a rise in viscosity, the idea of adjusting the pH value of the sol may be conceived as a countermeasure. The effect of this measure, however, is not sufficient.
A porous mold to be used for precision casting is generally manufactured by fabricating a casting model (the term "casting model" will be used hereinafter in the present invention) in a desired shape with wax, naphthalene, a low melting alloy, urea, or foamed styrene which is removable by heat, with water, or with an industrial chemical such as solvent, i.e. a thermally or chemically removable material, alternately applying to the casting model a slurry produced by mixing a fine refractory powder with a caking agent (hereinafter referred to occasionally as a "molding agent") and refractory particles identical or not identical with the fine refractory powder mentioned above, drying the coated casting model thereby forming a mold (the term "green mold" will be used hereinafter in the present invention), then removing the inner casting model by a thermal or chemical treatment, and sintering the residual green mold.
To be used as the casting agent in the form of slurry, various ceramic type casting agents including those of the silica type, the magnesia type, the zirconia type, and the calcia type have been developed. When a metal of high activity such as titanium or a titanium alloy is to be cast, however, a cast product of highly desirable quality cannot be obtained because of a serious metal-mold reaction if the mold to be used is formed of a silica type casting agent which is generally used extensively in casting a high melting metal. The feasibility of a casting agent of the magnesia type, the zirconia type, or the calcia type which is inconspicuously reactive with such a highly active metal and is effective in curbing oxidation has been being studied. This casting agent is not satisfactory in terms of strength and thermal shock resistance of the mold and in terms of strength and thermal shrinkage of the green mold and is in need of various improvements.
As a caking agent for the manufacture of a mold to be used in precision casting of titanium or a titanium alloy, it has been known to the art that zirconium salts such as zirconium acetate and zirconium nitrate and zirconium alkoxides such as zirconium butoxide are available besides zirconia sol. As is clearly noted from the method for the manufacture of a porous ceramic mold to be used for precision casting, the caking agent fulfills a significant role in the imparting of strength to the green mold and the mold. The zirconia type caking agent heretofore known to the art has been incapable of bringing about any fully satisfactory result. For the purpose of improving dispersibility, flowability, and stability of the materials for the mold and enhancing strength of the green mold and the mold, the feasibility of using the caking agent in combination with a varying organic binder and a varying surfactant has been studied. The combined use brings about an improvement appreciable to a certain degree but not fully satisfactory. A desire has been expressed in the industry to develop a caking agent of highly satisfactory quality, particularly zirconia sol.
An object of this invention, therefore, is to provide a transparent zirconia sol, particularly a transparent high-quality zirconia sol of a high concentration, and a method for the production thereof.
Another object of this invention is to provide a high-quality zirconia sol which exhibits lasting stability without inducing rise of viscosity or gelation and which, used as a coating agent or an inorganic binder, for example, exhibit thermal and chemical stability and a method for the production of the zirconia sol.
Still another object of this invention is to provide a high-quality zirconia sol which is useful as a coating agent capable of imparting to a substrate various properties such as resistance to heat and resistance to abrasion or as an inorganic binder capable of producing high adhesive strength and a method for the production of the zirconia sol.
A further object of this invention is to provide a slurry for the manufacture of a porous ceramic product possessed of lasting stability.
Yet another object of this invention is to provide a slurry suitable for the manufacture of a porous ceramic mold.
A further object of this invention is to provide a porous ceramic product excelling in mechanical strength.
A still further object of this invention is to provide a porous ceramic mold useful for precision casting of titanium or a titanium alloy.